For some time, CRT manufacturers have used filters to obtain contrast enhancement for full color displays. Useful filters transmit light of frequencies corresponding to the phosphor emission frequencies and absorb all other unwanted light. This substantially reduces glare in full sun applications, e.g., when combined with a front surface AR coating. The most commonly used phosphors for color displays at present are P43 and P22. Their primary emission bands are 544 nm (P43) and 445, 525 and 630 nm (P22), etc. Thus, the optimum contrast enhancement filter would have transmission windows of any desired level around these three wavelengths and would be opaque at all other wavelengths.
The first types of contrast enhancement filters utilized were composite filters.
The major disadvantage of a composite filter is the high cost of fabrication. This arises from the necessity to prepare a high optical finish on each of the filter surfaces and because the laminate must be bonded together with optical cements to eliminate Fresnel losses. Another significant disadvantage is the increased weight of a composite filter compared to a single glass filter. A typical prior art composite filter is disclosed in U.S. Pat. No. 4,245,242.
Because of the high fabrication cost of composites, single glasses which combine all required transmission characteristics (as much as possible) have been developed, relying for contrast enhancement primarily on neodymium. Relevant prior art glasses are disclosed in U.S. Pat. Nos. 4,521,524; 4,769,347; 4,288,250; 4,376,829; 4,769,347; 3,143,683; 4,520,115 (concerned with X-ray absorption); 4,405,881; and 4,390,637.
In addition, such prior art glasses also incorporate various colorants such as CeO.sub.2, Sm.sub.2 O.sub.3, Fe.sub.2 O.sub.3, CoO, NiO, CuO, Er.sub.2 O.sub.3, MnO.sub.2, V.sub.2 O.sub.5, Cr.sub.2 O.sub.3, etc. for purposes of obtaining a desired color. The colorants are normally utilized in combinations. CeO.sub.2 is also well known as a solarization inhibitor. See, e.g., the above-mentioned patents. Various of these conventional ingredients are also known to absorb short wavelength radiation, e.g., UV radiation, e.g., CeO.sub.2, TiO.sub.2, etc.
It is usually most difficult to achieve a desired color in such glasses since the colorants are often incompatible and can adversely affect contrast efficiency and since the primary contrast enhancement component, i.e., Nd.sub.2 O.sub.3, itself is highly colored (purple).
Thus, it is often difficult to achieve the given combination of desired color and contrast enhancement properties for a particular end use, e.g., for the color space shown in FIG. 1.
x, y and z are defined as such by the Handbook of Colorimetry, Arthur C. Hardy, c. 1936, pp. 9-10 ##EQU1##